The Effects of Earthworm Functional Group Diversity on Earthworm Community Structure

The effects of earthworm functional group diversity on earthworm community structure

Cróna Sheehan 1, Laura Kirwan 2, John Connolly2 Thomas Bolger1

1 UCD School of Biological and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland

2UCD School of Mathematical Sciences, University College Dublin, Belfield, Dublin 4, Ireland

Summary - A comprehensive understanding of how species/functional group interactions determine population dynamics, community composition and their effect on ecosystem functioning is important. This paper presents data from a mesocosm experiment, based on the simplex design, to examine the effect of interactions between earthworm functional groups, food supply and initial overall biomass on community structure. All communities containing anéciques moved towards domination by anéciques. The survival of anéciques remained constant irrespective of initial conditions, with no effect of initial community structure, food supply or initial biomass. The proportional biomass of epigées increased when they were placed in communities dominated by anéciques. Initial overall biomass had a significant effect on the survival of endogées, with increased survival at low biomass. Juvenile production was significantly increased in communities that contained a higher initial abundance of epigées. The anéciques had significantly increased production of juveniles at lower levels of initial biomass. Overall, earthworm functional group diversity had an idiosyncratic effect on earthworm assemblage structure.

Keywords: Functional groups, earthworms, interactions, population dynamics, community composition, simplex design, initial biomass, food supply, relative growth rates, survival, number of juveniles

Introduction

Earthworms play important roles in determining soil fertility and structure (Darwin, 1881; Barley, 1959b; Lee, 1985; Shaw and Pawluk, 1986; Curry, 1988; Edwards and Bohlen, 1996, inter alia) therefore it is important to have a comprehensive understanding of how species/functional groups interact to affect population dynamics, community composition and the effect of earthworms on ecosystem functioning. Information on interactions between earthworm species or functional groups is limited (Abbott, 1980; Elvira et al., 1996; Butt, 1998; Curry, 1998; Dalby et al. 1998; Lowe and Butt, 1999) but it is obvious that positive and negative interactions occur.

Both intra- and interspecific/group competition have been shown to affect the survival, growth, maturation and cocoon production of earthworms and these effects have even been demonstrated between species from different functional groups (Elvira et al. 1996; Dalby et al., 1998; Butt 1998; Baker et al 2002; Lowe and Butt, 2002). Hamilton et al (1998) found that the epigeic Eisenia fetida unexpectedly had a negative effect both on weight gain and cast stability of Lumbricus terrestris and interpreted this as being due to the rapid growth rates of the epigeic species allowing it to out-compete the larger anecic species.

Competition also affects burrow systems and burrowing behaviour (Capowiez et al., 2000). For example, the burrow system of Lumbricus terrestris (anecic) was less deep in the presence of Aporrectodea caliginosa (endogeic), than in the single species treatments (Jégou et al., 2001). The burrows of Aporrectodea caliginosa were significantly deeper in the presence of Aporrectodea giardi compared to the single species treatment. These changes may have been due to earthworms foraging deeper into the soil for food but it could also indicate that facilitation was occurring and that Aporrectodea caliginosa was utilising the burrows made by Aporrectodea giardi for movement and/or that the below ground casts produced by Aporrectodea giardi might provide a food resource for Aporrectodea caliginosa.

These effects are usually interpreted as being due to competition for food resources; however, little research has been done on the influence of food resource availability on earthworm interactions. In fact, in one of the few studies, which have specifically addressed this issue, it was found that, while growth and maturation of hatchling Allolobophora chlorotica were negatively influenced by the presence of other earthworms, a positive interaction was detected between Allolobophora chlorotica and Lumbricus terrestris, with the latter promoting growth of the former. This suggested that the anecic Lumbricus terrestris produced a food source (casting) more palatable than the surrounding soil (Lowe and Butt 2003). In addition, in treatments where food was milled, growth rates of Lumbricus terrestris hatchlings paired with adult Lumbricus terrestris and Allolobophora chlorotica were similar. However, in treatments with unmilled food, the presence of adult Lumbricus terrestris markedly reduced juvenile growth rates of Lumbricus terrestris over their growth rates in the presence of adult Allolobophora chlorotica. This suggests that intra-specific competition that occurred between juvenile and adult Lumbricus terrestris was reduced when food particles were reduced (milled).

Thus both positive and negative interactions occur among earthworm species and these are affected by food supply and population density. In this study we use a mesocosm experiment to examine how interactions between earthworm functional groups affect earthworm community structure under differing initial densities and food supply.

Materials and methods

Experimental design

Because of the difficulties identified with many previous experimental designs (Wardle, 1999, Connolly 1986; Bolger, 2001; Connolly et al., 2001) a novel design, based on the simplex design, was used (Cornell, 1990, Ramseier et al., 2005) In this design, communities were set up as either monocultures (endogeic, epigeic or anecic), centroids, consisting of equal amounts of each functional groups, binary mixtures, consisting of equal biomass of two functional groups or complete mixtures consisting of all functional groups, where the proportional contributions of the groups sum to one (Fig. 1).

The experiment was carried out at two densities of earthworm, 1.7g and 3.4g per mesocosm and at two levels of food supply (0.32g and 0.64g of ground dried grass per week), giving four combinations of biomass and food supply and a total of 52 communities.

Experimental unit

The experimental units consisted of Plexiglas cylinders (15cm diameter x 30cm depth) containing 2.65 litres of soil. The base of each soil column was fitted with 2mm mesh to prevent earthworms escaping.

The soil used in this experiment was topsoil obtained on the campus of University College Dublin. Initially the soil was hand sorted to remove macrofauna, large particles and plant fragments. It was then sieved through a 4mm mesh followed by a 2mm mesh and screened for any remaining visible organic fragments. After the soil had been placed into the units described above, 1 litre of distilled water was applied to each and this was allowed to settle for a day. After this, 2 x 250mls applications of soil suspension were added to boost the microbial biomass within the soil. Damp muslin cloth (10cm x 10cm) was placed on the soil surface to act as a non-digestible refuge for the earthworms particularly the epigées. Dried milled grass (Lolium perrene) and 300 mls of distilled water were added to the columns every 7 days.

Adult earthworms were obtained from a meadow in University College Dublin and sorted into three distinct functional groups based on Bouché (1977). The epigées were Lumbricus rubellus, Lumbricus castaneus and Satchellius mammalis, the anéciqueswere Lumbricus friendi and Aporrectodealonga and the endogées were Aporrectodea caliginosa, Octolasion cyaneum, Allolobophorachlorotica and Aporrectodea rosea. The earthworms were allocated randomly to experimental units.

Earthworm analysis

The individual weights of the earthworms added to the experimental units were recorded at the beginning of the experiment and weights were again recorded when the experimental units were being dismantled. Thus, survivorship and growth could be examined.

Statistical Analysis

Assessing factors that affect changes in Community Composition

The composition of a community will change if the relative growth rates of functional groups differ. Therefore, the differences in relative growth rates between functional groups in a community should be analysed in order to determine the factors that affect changes in community composition (Connolly and Wayne 2005, Ramseier et al, 2005). If there are s functional groups in a composition, and one group is selected as a reference, there will be s-1 of these relative growth rate differences (RGRD’s).

For this three functional group experiment, anéciques were chosen as the reference and so the response variables are RGRDAN-EPI and RGRDAN-ENDO. Each RGRD is modelled as a linear function of the explanatory variables, which include the proportions of each functional group in the initial composition (AN, EPI and ENDO), initial stand biomass and food supply where low levels are coded by 0 and high levels by 1. (biomass and food are centred in all analyses)

This is the simplest model and could be generalised to include interactions between biomass and food or proportion of functional groups. These interactions were tested and none proved to be significant and therefore the simple model as above was used.

Functional group effects are assessed through the coefficients of the functional group proportions in the model: a1is the differential (in the limit) between the growth rates of epigées and anéciques as the assemblage tends towards a monoculture of anéciques. Thus, a large positive value of a1 means that anéciques are favoured over epigées in assemblages dominated by anéciques.

Differences in coefficients reflect the effects of neighbours. If a2 = a1 the RGR difference between epigées and anéciques is unchanged by increasing the initial proportion of epigées at the expense of anéciques. Under the same change in initial proportions, a2 > a1 favours anéciques over epigées.

The effect of a change in initial endogeic proportion on the relationship between anéciques and epigées is measured through a3.

The effects of initial assemblage biomass and of the amount of food added on the relationships between functional groups are measured through the a4 and a5coefficients.

Survival of Earthworms

Factors that affect the proportion of individuals surviving (Psurv) in each functional group were investigated using logistic regression (McCullagh and Nelder, 1989). The proportional biomass of the three functional groups that were present in the initial community are denoted AN, EPI and ENDO respectively. Model (1) is the simplest logistic regression model. It assumes that the logit of Psurv , i.e. Ln (Psurv /( 1- Psurv )), is affected just by the linear effects of initial functional group proportions, the level of food supplied and the overall initial biomass in the community.

(1)

β1, β2and β3 represent the linear mixing effects of the functional groups. They are the predicted responses of the three functional groups in monoculture at the low levels of food and biomass. A linear combination of these gives the response of a mixture of species. β4and β5 represent the effects of food and biomass. This is a constant shift in response at high levels of density and factor. Terms reflecting the pairwise interactions between functional groups were added to model (1) and tested for inclusion in a final model for each functional group. Values of the response variable were predicted from these final models and back transformed to the proportional scale.

Production of Juveniles

The number of juveniles produced by a community is a non-negative integer and so cannot be modelled by ordinary least squares regression. It is assumed that the number of juveniles, Y, has a Poisson probability distribution, and that its mean, μ, is related to initial community structure (McCullagh and Nelder, 1989). Poisson regression was used on these data. The simplest model is as follows: The expected value of Y is  and

(2)

The log link function ensures that the mean number of juveniles for each community predicted from the fitted model is positive.

Results

Biomass and proportions of earthworms

The proportional biomass of the earthworm functional groups changed during the experiment (Fig. 2). For each of the four combinations of food and biomass, virtually all communities containing anéciques moved towards domination by that functional group (Fig 2). The relative growth rates of the different functional groups were affected by various aspects of community structure and resource availability. The biomass of anéciques always increased but the higher the proportion of the other two functional groups present, the greater was their relative growth rate (Table 1). However, the biomass of epigées declined as the proportions of either epigées or endogées increased. As the biomass of earthworms in a community increased the relative growth rates of endogéics declined.

Community composition of earthworms

The parameter estimates for the two RGRD models are given in Table 2. The positive coefficients for the epigeic (EPI) and endogeic (ENDO) imply that the growth rate of the anéciques, relative to these groups, increases as the proportions of these functional groups in a community increase and reflects the greater proportions of anéciques at the end of the experiment. As the effects of food supply and initial biomass were not significant, a single graph can be used to predict the changes in community composition over the period of the experiment (Fig. 3). Almost all communities move towards domination by anéciques.

Survival

The survival of anéciques remained constant irrespective of initial conditions. There was no effect of initial community structure, food supply or initial biomass. The average survival rate was 73%.

Survival rate of epigées was very low in communities dominated by either epigées or endogées (Table 3, Fig 4). However, increasing the proportion of anéciques in the community increases the rate of survival (Fig. 4). For example, the survival of epigées in monoculture was 26% while in the community with 80% anéciques it was 86% at average biomass and food supply. Antagonistic effects of mixing anéciques and epigées (p=0.031) and anéciques and endogées (p=0.003) ensured a low level of epigée survival where anecic proportion was less than about 0.5 (Fig 4). These outcomes were not influenced by initial biomass or food supply.

The survival rate of endogées was significantly higher in communities that contained a higher initial abundance of endogées (c.f. positive coefficient on the initial proportion of endogées in the community) (p<0.001) (Table 4). Endogeic survival was low in communities dominated either by epigées or anéciques but was quite high when these species were equally represented (Fig. 5) due to a positive synergistic effect between them (Table 4). The endogées had higher survival rates at lower overall initial earthworm biomass in the community (ENDO*BIOMASS interaction; p=0.001) (Fig. 5). For example, the survival of endogées in monocultures at high biomass was 62% and 90% (high biomass low food supply and high biomass high food supply respectively) while at low biomass it was 100% (for both low biomass low food and low biomass high food). Food supply did not have a significant effect on the survival of endogées (p=0.2289).

Juvenile production

The production of juveniles increased in communities that contained a higher initial abundance of anéciques as indicated by the positive coefficient for the proportion of anéciques (p=0.0015) (Table 5). The production of juveniles was significantly increased when the anéciqueswere mixed with endogées (anéciques*endogées interaction; p=0.025) (Table 5, Fig.6).

Food supply significantly affected the production of juveniles. In particular there was a significant interaction between the proportion of epigées and food supply and large numbers of juveniles were present in monocultures of epigées at high food supply (p=0.049) (Fig. 6 and Table 5). There was also a larger production of juvenile epigées at the lower initial community biomass (p=0.049). With high biomass and low food supply, a significant difference was detected between the epigées and anéciques (p=0.0265) and anéciques were predicted to produce 2.86 more than the epigées (Fig. 6). At low biomass and high food supply, epigées were predicted to produce 7.3 more juveniles than the endogées (p=0.0211) (Fig. 6).

Discussion

A comprehensive understanding of how functional group interactions determine population dynamics, community composition and their effect on ecosystem function is important. The coexistence of species/functional groups in the same habitat can lead to interactions, which can have a positive or negative effect upon them. Although interactions between organisms are generally well documented (Connell, 1983; Gurevitch et al., 1992), little work has been done on interactions among earthworm species/groups (Curry, 1998; Dalby et al., 1998; Lowe and Butt, 1999).

In this study all communities containing anéciques, even those initially dominated by either of the other groups, became more dominated by this group as the experiment progressed.. In fact the relative growth rate of the anécique population was increased as the proportion of epigées and endogées increased. However, they became less dominant in communities containing the large initial biomass of anéciques perhaps indicating competition for food and/or habitat. The relative growth rates for epigées on the other hand indicate that they are significantly negatively affected by increased biomasses of the other two groups.

Similar trends were observed with survival. The survival of the anéciques remained constant irrespective of initial condition. There was no effect of initial community structure, food supply or initial biomass on their survival. There was evidence of intra-group competition or lack of required synergistic effects occurring in communities dominated by the epigées, as their survival was found to be significantly lower in these communities. The results imply that the survival of the epigées may be directly related to optimal habitat availability. As with the proportional biomass, the survival of the epigées was greatly increased as the initial proportion of the anéciques in the community increased. This suggests that a positive inter-group interaction was occurring.

Despite the very poor survival of endogées in communities dominated by either of the other two groups, the greatly enhanced survival where these species are in balance is intriguing. The anéciques may have provided a food resource to the endogeic earthworms by depositing faecal organic matter (or casts) into the soil where the endogées can reach it. The epigées play a major role in organic matter comminution and reduce the size of organic matter particles during the passage through the earthworm and in doing so makes organic matter more assessable to digestive action by other decomposers. Endogées prefer feeding on well decomposed organic matter. Hence these combined effects may have played a role in the improved survival of these earthworms. In communities dominated by endogées their survival was reduced at high biomass suggesting intra-group competition.

The production of juveniles was significantly increased in communities that contained a higher initial overall abundance of anéciques. This is presumably correlated with the high rates of growth and survivorship of this group. However, there was also an increased survivorship of epigées under these circumstances and, although the biomass of this group may have been low under these circumstances, the smaller adult sizes of these worms and the fast rate of development of epigeic cocoons could have contributed to the increased production of juveniles (Senapati & Sahu, 1993; Bhattacharjee & Chaudhuri, 2002). Increased functional diversity led an increased production of juveniles, as indicated by the positive interaction between the anéciques and endogées, but food supply and biomass proved to be important for the epigées, with increased juvenile production at high levels of food supply and at lower levels of initial overall biomass suggesting that the higher number of juveniles occurred where there was reduced competition for food and space or that the additional food supplied at the surface of the soil provided an appropriate habitat for this group..